<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        @keyframes ani {
            from {
                background-color: red;
            }
            to {
                background-color: blue;
            }
        }
        div {
            width: 200px;
            height: 200px;
            border: 1px solid #000;
            margin: 50px auto;
            animation: ani 3s;
        }
    </style>
</head>
<body>
    <div></div>
</body>
</html>